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LM13700 Filter by Look mum no Computer PROBLEM
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Manuel Sammet



Joined: Sep 04, 2019
Posts: 4
Location: Augsburg

PostPosted: Wed Sep 04, 2019 2:23 pm    Post subject:  LM13700 Filter by Look mum no Computer PROBLEM
Subject description: (issue)
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Hello People,

I wanted to rebuild the lm13700 based stripped down Filter, introduced by "look mum no computer".

https://www.youtube.com/watch?v=q-2MkTAr4xk&t=355s

homepage with instructions:
https://www.lookmumnocomputer.com/simple-filter

To mention: There are no Schematics of his pretty easy and simple Filter, based on Rene Schmitz design - only the video and the tutorial on his homepage.

I use a Meanwell rt-50b which provides +12vdc, -12vdc and grounding.

I build all exactly like he did on his stripboard. cutoff works, cv voltages for the cutoff work as it should but resonance seems to be pretty high. only in selfoscillation i hear a chirping. no wetting at all while turning the resonance pot up. i replaced it by a 100k log-a pot, but nothing changed it.

above you can see pictures of my project and in the beginning u see his video and stripboard layout. i really tried all i can but i have no clue how i can fix the resonance problem.

any ideas?

it was supposed as a birthday present for a good friend this saturday. if someone might be able to help me, i can send more informations.

i calibrated the output voltages to a 1000th with my table multimeter. while powering the filter with it the in and output voltages on the tl074 and lm13700 are exact the voltage that comes out from my powersupply.

i doublechecked myself if i did anything wrong, but i couldnt find a error in my own thinking...maybe theres something i did wrong but i have no idea.

someone knows how to make the resonance wetting my signal normally?

best regards,

Manuel.


EDIT:

I regoniced i used 1.2nf c-caps instead of the 1.0nf c-caps, because my vendor hasnt the ones. can this be a problem?

https://ibb.co/2qzYRGY
https://ibb.co/3T3c4Yg
https://ibb.co/XjLfPbJ
https://ibb.co/XjLfPbJ
https://ibb.co/By5v7Tm
https://ibb.co/NxxPm3P
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Grumble



Joined: Nov 23, 2015
Posts: 1294
Location: Netherlands
Audio files: 30

PostPosted: Thu Sep 05, 2019 12:38 am    Post subject: Reply with quote  Mark this post and the followings unread

The picture shows no ground connection to the bottom left connector and it looks like you havent done this also.
LMNC assumes that your connectors are mounted on an aluminum face panel.
So as long as this connector is not mounted together with the other connectors on an aluminum faceplate, you need to add a ground connection to this connector.


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Manuel Sammet



Joined: Sep 04, 2019
Posts: 4
Location: Augsburg

PostPosted: Thu Sep 05, 2019 2:55 am    Post subject: Reply with quote  Mark this post and the followings unread

yes, i had a humming all over my output signal, so i grounded the input. no noise anymore - but the resonance problem appeared Sad
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Grumble



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PostPosted: Thu Sep 05, 2019 4:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Maybe HERE some answers for you?

Or HERE?

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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Thu Sep 05, 2019 8:35 am    Post subject: Re: LM13700 Filter by Look mum no Computer PROBLEM
Subject description: (issue)
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welcome party!

Quote:
To mention: There are no Schematics of his pretty easy and simple Filter, based on Rene Schmitz design - only the video and the tutorial on his homepage.

let's fix that first.
I compared the stripboard layout with the Rene Schmitz version and it is the same apart from using an LM13700.

It does have pretty high gain in the feedback path so yes it will self oscillate with high resonance which results in the chirping sound.
I am not sure how wet this one can sound and it depends a on the input signals you use.


Quote:
I regoniced i used 1.2nf c-caps instead of the 1.0nf c-caps, because my vendor hasnt the ones. can this be a problem?

shouldn't matter much as long as you don't use different values for each capacitor. Ideally they should be matched.


Rene Schmitz MS20 filter (LM13700).gif
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Last edited by PHOBoS on Thu Sep 05, 2019 1:35 pm; edited 1 time in total
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Manuel Sammet



Joined: Sep 04, 2019
Posts: 4
Location: Augsburg

PostPosted: Thu Sep 05, 2019 9:45 am    Post subject: Reply with quote  Mark this post and the followings unread

You mean the resonance-path, from pin3 of the 074 to the 100k trimpot? what bothers me while looking on the attached picture from you is that pin3 is an non-inverting input? maybe this is the problem? or whill make a resistor before the 100k resonance-pot a difference?

im using an arturia microbrute to test the filter.

im in my garage now, trying to figure out what it is.
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PHOBoS



Joined: Jan 14, 2010
Posts: 5591
Location: Moon Base
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PostPosted: Thu Sep 05, 2019 11:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Opamp U2a is configured as a non-inverting amplifier with a gain of about 6.5x. Adding a resistor before the 100K pot
(that is between the output and the pot) would have the same result as not turning up the resonance pot completely.
This can be useful though, if you add a trimpot you can set the maximum resonance, however that doesn't really affect
how it sounds. I did watch the video again (I actually had just watched it a few days ago) and noticed he used red LEDs
You could try green LEDs which should give the opamp a larger possible output swing but that might sound worse.

The Arturia MicroBrute is more than capable of creating some rich waveforms to test with so that's fine.

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Manuel Sammet



Joined: Sep 04, 2019
Posts: 4
Location: Augsburg

PostPosted: Thu Sep 05, 2019 12:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

ok i try this. actualy im following the schematic and test if i 100% wired all correctly. i soldered out the resistors and tested them again, but theyre fine. ill reply asap i tested this with the leds.

is this a error or why are the bc558`s drawed as npn transistors?
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PHOBoS



Joined: Jan 14, 2010
Posts: 5591
Location: Moon Base
Audio files: 705

PostPosted: Thu Sep 05, 2019 1:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Manuel Sammet wrote:
is this a error or why are the bc558`s drawed as npn transistors?

Embarassed my mistake,. I'll change it ASAP.

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cyclic



Joined: Mar 15, 2015
Posts: 95
Location: hobart

PostPosted: Wed Aug 25, 2021 5:55 pm    Post subject: Re: LM13700 Filter by Look mum no Computer PROBLEM
Subject description: (issue)
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PHOBoS wrote:
welcome party!

Quote:
To mention: There are no Schematics of his pretty easy and simple Filter, based on Rene Schmitz design - only the video and the tutorial on his homepage.

let's fix that first.
I compared the stripboard layout with the Rene Schmitz version and it is the same apart from using an LM13700.

It does have pretty high gain in the feedback path so yes it will self oscillate with high resonance which results in the chirping sound.
I am not sure how wet this one can sound and it depends a on the input signals you use.


Quote:
I regoniced i used 1.2nf c-caps instead of the 1.0nf c-caps, because my vendor hasnt the ones. can this be a problem?

shouldn't matter much as long as you don't use different values for each capacitor. Ideally they should be matched.



Alrighty then folks, and specifically Senior Advanced Respected Phobos:

I have been looking about for a simple filter SPECIFICALLY for circuit bent toys and instruments (ie NOT MODULAR!)
I want something small and simple and I NEED something that will run at 4.5v (ie 3xAA batts as often in toys)

I have some oldstock low voltage opamps that will run down to +-2V (M5218) and have some even lower power ones coming which I think do rail to rail from +-1.5v.

Do you reckon this circuit will need much fandangling to work at such low voltages?

Perhaps change the LEDs to red or even a diode? I believe they are actually here as waveshapers/soft clippers or something, NOT as light sources...

Perhaps different biasing around the PNP pair? Transistor biasing sares and confuses me so I have no idea what to do there...


To forestall other possible input: Yes I have a Wasp filter already. It keeps eating my 4069UBEs which must be shitty quality as they are a bit harder to get cheaply, and the sound is never as awesome as I think it should be. PLUS really, I want to smooth the Bent instruemnts, not roughen em.,..


cheers,

Lance
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AlanP



Joined: Mar 11, 2014
Posts: 746
Location: New Zealand
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PostPosted: Thu Aug 26, 2021 12:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

https://electro-music.com/forum/topic-67769.html

Lance, this might be better for you.
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cyclic



Joined: Mar 15, 2015
Posts: 95
Location: hobart

PostPosted: Thu Aug 26, 2021 7:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

AlanP wrote:
https://electro-music.com/forum/topic-67769.html

Lance, this might be better for you.


Ah yes. I'm well aware of the Q+D and have used it in the past.

I should have been clearer that I'm after a less simple one than this, more of a 'proper' filter than a quasi-filter...

... but since you have brought it up I might revisit it as its been 5 years or so...
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cyclic



Joined: Mar 15, 2015
Posts: 95
Location: hobart

PostPosted: Thu Aug 26, 2021 7:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

yeah. ok. I definitely built the first 2 of that, nto hte two-diode version and the video linked to there shows that the 3rd is indeed a LOT better, isnt it....

thanks for encouraging me to review a discarded idea given that other people have updated it!
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